Cobb's "Cool" Medieval Pacific

I noted yesterday that Allen [2006] did not contain any new data supporting the idea of a "cool medieval Pacific", but that it was based primarily on a paper by Kim Cobb et al [2003] based, as it turns out, on two individual corals at Palmyra Island (6N) – one dated to the 10th century and one to the 12th century – both dated done by U/Th measurement, which show relatively elevated dO18 anomalies, while modern corals show relatively depressed dO!8 anomalies in the late 20th century. On a quick read, I’ve got several caveats about dating and replication. However, more fundamentally, Cobb’s recent sampling and interpretation of dO18 values at Christmas Island at 2N makes me wonder whether these results can’t be readily interpreted as evidence of a more northerly ITCZ in the 10th century, which would be a very interesting result and easily consistent with medieval NH warmth.

I met Cobb a couple of years ago at the AGU conference. In addition to working on corals, she was sampling speleothems in Borneo. I’ve gone into some abandoned adits in Chile for short periods and never much liked the bats and snakes, so I chatted briefly with her about such problems. The coral islands that she samples are spectacular looking.

Here’s Cobb’s argument. dO18 in tropical precipitation is governed by an amount effect – lower dO18 anomalies occur in summer, not winter; more negative dO18 anomalies are associated with El Nino. Note that she is associating more negative dO18 with higher temperatures (the scale is reversed in the graph shown later) while Thompson at tropical ice cores associates more positive dO18 anomalies with higher temperatures.

Climate at Palmyra is dominated by ENSO variability, manifest as warmer, wetter conditions that persist during El NinËÅ“o events and cooler, drier conditions during La NinËÅ“a events (Fig. 1a). Positive sea surface temperature (SST) and rainfall anomalies that occur at Palmyra during an El NinËÅ“o event both result in lowered (more negative) coral d 18O, while the converse is true during a La NinËÅ“a event. Thus, Palmyra corals are sensitive recorders of regional-scale ENSO activity. For example, a d 18O record from a modern coral (Porites lutea) at Palmyra shares 72% of its interannual variance with the NINËÅ“ O3.4 index (Fig. 2), demonstrating that coral records from this site provide reliable proxies of ENSO activity, at least over the twentieth century23. Our ability to reconstruct ENSO from fossil corals at this site rests in part on the assumption that the spatial patterns of ENSO have not changed significantly over the last millennium. (NAture)

Her main graphic from Cobb et al (Nature 2003) – the one that’ relied on for the "cool medieval PAcific" – is shown below. The top panel undoubtedly looks familiar – it’s Mann’s hockey stick which supposedly has not influenced other studies. The second panel shows dO18 levels from Palmyra island corals. Two things: the corals are from continuous. Modern corals only go back to the 19th century – fossil corals are dated to 4 other intervals: 17th century; 14-15th century, 13th century and 10th century. The theory of a "cool medieval period" arises from the less negative dO18 anomaly for the coral dated to the 10th century (and the more negative dO18 anomaly for the living coral – again take care with the reversed y-axis scale.)

Original Caption: Figure 5 Comparison of proxy climate records and external forcing during the last millennium. a, The MBH Northern Hemisphere temperature reconstruction34 (green) plotted with the Northern Hemisphere instrumental temperature record of ref. 48 (red). The green horizontal line denotes the mean of the MBH record for the period AD 1886″€œ1975. b, The monthly resolved Palmyra corald 18O records (thin black line), shown with a 10-yr running average (thick yellow line). The black horizontal line represents the average of the Palmyra modern coral d 18O for the period AD 1886″€œ1975. The black vertical bar represents the^1j error in mean corald 18O for single fossil corals (this error applies only to tenth- and twelfth-century sequences). The dating error is ^10 yr for the tenth- and twelfth-century sequences, ^5 yr for the fourteenth”€œfifteenth- and seventeenth-century sequences, and^,0.5 yr for the twentieth-century sequence. c, Reconstruction of solar irradiance anomalies based on historical sunspot records (anomalies calculated with respect to the AD 1886″€œ1975 mean)49 (purple) plotted with 10Be anomalies (a proxy for solar activity)50 (blue), plotted as a 3-point running mean and scaled to the solar irradiance anomalies. d, Radiative forcing associated with volcanic eruptions recorded in ice cores (black)35. The approximate timing and duration of the “‘œLittle Ice Age’ (LIA), the “‘œMedieval Warm Period’ (MWP), and solar activity minima”€?the Maunder minimum (MM), the Spo¨ rer minimum (SP), and the Wolfe minimum (WM)”€?are marked by horizontal bars.

Dating
Dating is done through U/Th dating as outlined in Cobb et al (EPSL 2003a), relying ultimately on the ratio of 230Th to 238U to date the corals. The table shown below shows her sample results – you will see that the ratios here fall into two main groups – and, in fact, cover only the modern corals and the 13-14th century corals. The "cool medieval" corals are not shown in this table, but presumably have even higher 230Th/238U ratios.

I find the lack of continuity in the dates to be disquieting – I realize that sampling was catch-as-catch-can, but why would there be a concentration of 14th century corals and a seeming absence of 18th century corals? Note that there is only one coral for the 12th century and one for the 10th century, so these results are not well-replicated.

Cobb also notes that natural thorium is present and that this has the effect of making the samples seem older than they really are. She considers a max and a min-adjustment, with the min-adjustment using a 230Th/232Th ratio of 2.4*10^-6, which is the value at secular equilibrium with earth’s crustal ratio of 232Th/238U. Her max correction is 2.0*10^-5, which is said to be the "highest value required to correct samples in the present study". I can’t see where this value is calculated. In her text, she discusses some 230Th/232Th measurements at the island: seawater values 1000 km from Palmyra were 5-10*10^-6 in surface water and 2*10^-4 in deep water, noting that upwelling could affect Palmyra’s seawater. She also notes that carbonate sands produced by ongoing erosion of the coral reef may contribute Th with 230Th/232Th ratios as high as 1*10^-2, in the case of 5000 year old corals. This value is obviously much higher than the values used in Cobb’s maximum correction. I don’t know enough about dating issues to comment further – these are merely issues that caught my eye.

ITCZ Movement
In discussions of the Cariaco (Venezuela) time series, Haug et al 2003 convincingly associate changing values with north-south movements of the ITCZ. While I’m far from having fixed positions on such matters, it seems like a plausible proximate cause for many centennial and millennial effects, although no consensus is reached on explaining centennial changes in ITCZ location (precession undoubtedly affects ITCZ location on Milankowitch scales). Cobb refers to the ITCZ in her most recent presentation.

If you re-examine the above information in light of her most recent results at nearby islands, I wonder if what we’re seeing here might be more resaonably explained by north-south ITCZ movements (noted elsewhere.) First, here’s the location of the three islands sampled by Cobb – although only the most northerly Palmyra is reported in the 2003 articles. The color coding shows El Nino effects.

Caption: During El NiàƒⰯ events, positive SST and precipitation anomalies both contribute to negative coral àŽⲱ8O anomalies in the CTP Interpretation of coral àŽⲱ8O on lower frequencies relies on assumption that warm SST drives higher precipitation in the CTP, and vice versa SST and rainfall anomalies during the 1982 El Nino

Next here is a very interesting figure from Cobb’s recent presentation at Florida State. Note that the dO18 anomalies at Palmyra (6N) are more depleted than at Christmas Island (2N). I’m not sure what she expected – but intuitively if the relationship was that more negative dO18 was associated with El Nino, wouldn’t you expect to see more negative dO18 values at Christmas Island? In fact, her results are the opposite! It’s too bad that a modern sample wasn’t taken yet at Fanning Island, but these are not easy places to get to, and one can hardly quibble.

If you re-examine the values in the top figure, you will see that the 10th century values at Palmyra – supposedly showing a "cool medieval Pacific" are in the -4.1 to -4.2 range – i.e. in the range of modern corals at Christmas Island about 200 miles to the south. Assuming that the corals have been dated correctly, isn’t it quite possible that the ITCZ in the 10th century was about 150-200 miles further north, thus yielding dO18 values at Palmyra corresponding to Christmas Island today? As I read it, the still unpublished dO18 coral values from Christmas Island completely overturn the cool medieval Pacific theory insofar as it was based on less negative dO18 anomalies at two Palmyra Island corals dated to the medieval period.

1. The uptick in O18 is only over the last 20 years. Why is there no uptick from 1900-1980?

2. After taking first differences, these two series (temperature, O18) will probably not correlate at all. i.e. The correlation in the undifferenced data is due to a shared trend, which is not statistically significant when you correct for loss of effective degrees of freedom due to serial autocorrelation.

I’m still looking at her publications (available on her site). Some things I’m trying to understand.
1. Does she report or record all the corals found? Does she analyze them all? (I’m interested in general variability within the group). Also is the data archived and if so, how?
2. Is there any wiggle-matching? Should there be?

“In total, the analysis thus far shows a clear “regime-like” behavior of ENSO. This non-stationarity is evident throughout the record, and the characteristics of ENSO thus far show little correlation with other indicators of global climate.”

Title: Lake sediments record large-scale shifts in moisture regimes across the northern prairies of North America during the past two millennia

(Six high-resolution climatic reconstructions, based on diatom analyses from lake sediment cores from the northern prairies of North America, show that shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia.

These large-scale shifts at the different sites exhibit spatial coherence at regional scales. The three Canadian sites record this abrupt shift between anno Domini 500 and 800, and subsequently conditions become increasingly variable. All three U.S. sites underwent a pronounced change, but the timing of this change is between anno Domini 1000 and 1300, thus later than in all of the Canadian sites. The mechanisms behind these patterns are poorly understood, but they are likely related to changes in the shape and location of the jet stream and associated storm tracks. If the patterns seen at these sites are representative of the region, this observed pattern can have huge implications for future water availability in this region. )

***Abrupt changes in climatic conditions from the semiarid prairie region in northern North America indicate that shifts in drought regimes on a multicentennial scale are a common feature of this region. Regardless of the exact timing of these shifts or the direction of these changes, the persistent and abrupt nature of these events represents a scale of variation that is not well understood. Such shifts today would prove a major challenge for society regardless of global warming (42), particularly given that persistent periods of drought have been shown in numerous cases to coincide with societal stress and collapse (2). ***

In total, the analysis thus far shows a clear “regime-like” behavior of ENSO. This non-stationarity is evident throughout the record…

I’m not sure I agree with this interpretation. A cursory examination of the time series (in the time domain) suggests a complex process with interesting time-series structure (long-term persistence?), but this is in no way the same as non-stationarity. Is Cobb working with a competent statistician?

#8. MarkR (and this might also interest Rocks) – if you look at bathymetric maps of the Pacific, you’ll see that there are very few channels for Antarctic Deep Water and the most important one is around Samoa. Given the amount of tectonic activity at Samoa, I wonder whether this has affected northward flow of Antarctic Deep Water in the Tertiary and what impact this would have.

“on two individual corals at Palmyra Island (6N) – one dated to the 10th century and one to the 12th century – both dated done by U/Th measurement, which show relatively elevated dO18 anomalies, while modern corals show relatively depressed dO!8 anomalies in the late 20th century.”

My impression is that they are mostly fossil corals recovered from the beach). I think it would be interesting if she started working with the smaller corals (pebbles and such) allowing her to take more samples, more quickly by doing away with that pesky pump. Would need to do some statistics to handle the extra load of data and the increased uncertainty from not having wiggle-matching.

#18. No, they’ve published their articles and the articles are in the public record. If they wish to reply here, I am happy to prominently display their reply as a separate post with equal stature to my original comment and have done so in the past.

Steve, it’s an interesting point. I agree that you don’t need to notify an author you are critiquing. I also don’t think there would be any point with the hockey team, as they aren’t going to respond.
But with non-hockeyists, as here, it’s possible it could encourage them to engage on the issues. (I’m not ready to abandon the optimistic assumption that there are still some real scientists out there somewhere.)
Just a thought …

People, this – the toughest point in the introductory discussion – is hardly a scathing “critique”:
“I wonder if what we’re seeing here might be more resaonably explained by north-south ITCZ movements”

It is an alternative interpretation, a conjecture, posed interrogatively no less. You can’t be much more polite than that.

It is just not customary to write letters to authors asking them to justify their interpretations. In fact, it is not even the role of an audit (where the focus is on methods and results). What IS customary is to ask them the question in person should you encounter them giving a talk in a public forum.

Re #22, bender
Surely customs are a matter of the past, from before these new-fangled interweb blog thingies. Wild crazy cool cats like Steve are creating the new customs for the new millenium.
All I was suggesting was an e-mail saying “we’re about to discuss your paper in a public forum, see link. Do feel free to join in if you feel so inclined”.

# it’s a reasonable suggestion and I’ll probably do it. It’s probably a good idea with other people as well – I don’t think that I’m obligated to do so, but it’s hard to see any harm in it. I suspect that more people will encounter her 2003 EPS&L article through the blog reference than ever read it in the first place.

Surely you are not obligated to do so Steve. But when a lot of examining what an author did or meant to say a two way dialogue could go a long way rather then just guessing what she/he meant by such and such a statement.
For instance the opposite use of the 18O ratios must/might have a simple explanation the author could easily clarify.
[snip]

Uranium-thorium measurements make it possible to obtain windows of shorter records within the last few millennia to shed light on tropical and subtropical ocean climate variability. Cobb et al (2003) reported a àŽⲱ8O record for portions of the last 1,100 years from fossil corals at Palmyra atoll in the mid-tropical Pacific. As shown in Figure 5-3, they find relatively cool and dry climate conditions during the 10th century to increasingly warmer and wetter climate in the 20th century. ENSO activity was found to be most intense during the mid-17th century than during other periods examined. Although not a problem with these data, the possibility of diagenetic alteration of fossil segments must be considered in coral studies.

Re #28:
Read #3 again, and ignore the later typo in #19 (where I typed 1980 instead of 1900). What I asked was “why is there no uptick from 1900-1980?”. And again, for a third time I ask: where’s the correlation analysis with differenced time-series?

It would be interesting to know exactly where the samples were taken, and whether they were random, also is this atoll and its environs still (in recent geological timescales) volcanically active in any way, any vents down the sides etc.

Here’s Cobb’s argument. dO18 in tropical precipitation is governed by an amount effect – lower dO18 anomalies occur in summer, not winter; more negative dO18 anomalies are associated with El Nino. Note that she is associating more negative dO18 with higher temperatures (the scale is reversed in the graph shown later) while Thompson at tropical ice cores associates more positive dO18 anomalies with higher temperatures.

When working with oxygen isotopes it is necessary to be careful to distinguish the archive you are working with. In the case of Cobb’s work she is measuring the delta 18-O composition of coralline carbonate. The carbonate is precipitated from sea water and the oxygen isotope composition of the carbonate is related to that of the seawater by a temperature dependent fractionation factor. i.e. the 18-O/16-O ration of the carbonate divided by the 18-O/16-O ratio of the seawater. As the sea increases in temperature the value of this fractionation factor decreases. The gradient is about -0.2 per mille per degree C. Thus when Cobb states that more negative values oif d 18-O record higher temperatures she is writing about the temperature effect on the carbonate-water oxygen isotope fractionation factor. I admit that her wording is ambiguous….she talks about higher SST’s and higher precipitation and conflates a temperature effect with an amount effect!

Thompson is working directly with precipitation samples and is appealing to a temperature effect being the dominant signal in his ice isotope record. I think he might well be wrong! The two dominant signals in precipitation are a temperature effect in high latitude precipitation, and a more dominant amount effect in the tropics. The temperature effect for precipitation is seen as an increase in the delta 18-O of precipitation with increasing temperature. This is the opposite of what one sees with carbonates and is the source of much confusion.

#38. Paul, thanks for this. I wasn’t unduly bothered by the fact of differing anomaly directions – although I am bothered by Thompson’s cavalier assumption that less negative dO18 anomalies in ice cores can be attributed to rising temperatures in his context.

The observation that was really on my mind – and I should have bit my tongue on the extraneous comment – is that the coral anomalies at Christmas Is (2N) are less negative than the coral anomalies at Palmyra Island – and that dO18 levels of the “MWP” coral at Palmyra Island are directionally similar to the dO18 levels of modern Christmas Island. The dO18 levels at Christmas Island is brand-new data – unreported as far as I know except in Cobb’s presentation to a Florida State seminar. I think that the possibliity of N-S movements of the ITCZ provides a rather neat explanation of the phenomenon.

It’s intriguing how quick people have been to seize on such slight evidence for a cool medieval Pacific.

“I think that the possibliity of N-S movements of the ITCZ provides a rather neat explanation of the phenomenon.”
.
Perhaps Judith Curry would care to comment on the possibility of an ITCZ shift northward in 2005 and a possible link between (1) hurricane activity in the Gulf of Mexico (e.g. Katrina), and (2) drought-caused forest decline in the Amazon Basin? Both occurred in 2005.

Steve, I think you’ve picked up on an interesting idea about movement of the ITCZ here.

I’m surprised by the difference between Christmas Island and Palmyra Island. I think there is nearly a per mille difference between them implying that the ocean around Palmyra is about 5 degrees warmer at the present day.

What really concerns me about some of these studies is there is absolutely no reason for not collecting seawater for isotope measurement, and monitoring SST’s at the time of sample collection etc. then checking to see if the corals are precipitating calcium carbonate in, or close to oxygen isotope equilibrium with the ocean. I’ll wager several pints of beer, or your favourite tipple that they are not. Figures around -4 to -5 per mille for the carbonate isotope composition imply temperatures of 30 to 40 degrees C.

As a complete aside I’m off on vacation to sit and cogitate with copious amounts of wine for a few weeks. I will be back!

The ITCZ can move dramatically from week to week. It can also vary substantially from year to year. This year, the Pacific is experiencing a reasonably far north ITCZ. Last year it was further south. I automatically am suspicious of any claim that paleo measurements of its “average position” are all that meaningful.